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Bài I)

a) $\displaystyle A=7\sqrt{5} +3\sqrt{20} -\sqrt{125} -2\sqrt{45}$

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
=7\sqrt{5} +3.2\sqrt{5} -5\sqrt{5} -2.3\sqrt{5}\\
=7\sqrt{5} +6\sqrt{5} -5\sqrt{5} -6\sqrt{5}\\
=2\sqrt{5}
\end{array}$

b) $\displaystyle B=\sqrt{9( x-5)^{2}} \ ( x\geqslant 5)$

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
=3|x-5|\\
=3( x-5)
\end{array}$

c) $\displaystyle C=x-4+\sqrt{49-14x+x^{2}} \ ( x< 7)$

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
=x-4+\sqrt{( 7-x)^{2}}\\
=x-4+|7-x|\\
=x-4+7-x\\
=3
\end{array}$

d) $\displaystyle D=\sqrt{9+4\sqrt{5}} +\sqrt{9-4\sqrt{5}}$

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
=\sqrt{\left( 2+\sqrt{5}\right)^{2}} +\sqrt{\left( 2-\sqrt{5}\right)^{2}}\\
=2+\sqrt{5} +\sqrt{5} -2\\
=2\sqrt{5}
\end{array}$